This invention relates to the field of synchronization systems for televisions and the like, and in particular, to the generation of horizontal blanking signals for multiple scan rate operation. For example, the horizontal blanking signals are generated at 2f.sub.H, where f.sub.H is the conventional horizontal scanning frequency.
Proper width and deflection/video timing are important in achieving a video signal that does not have any fold over. This is especially true in high end receivers that run at multiple horizontal frequencies (nf.sub.H), and lower overscans. The amount of overscan for multiple frequency scanning is approximately 5% to 7%, as compared to the 10% to 12% overscan which is common for conventional scanning rates. Accordingly, the need is greater for accurate timing of the horizontal blanking signal.
Typically, horizontal blanking is derived from a lower voltage pulse on a secondary winding of the high voltage flyback transformer. A problem with this scheme is that the rise time of the pulse is not fast enough to blank adequately the video in the horizontal yoke current retrace interval which is generated by a higher voltage pulse. Moreover, unless the pulse is heavily differentiated and then stretched to make the pulse wide enough, the timing of the beginning of blanking is later than needed. Unfortunately, heavy differentiation creates other problems in the form of false triggering of the blanking circuit when flyback ringing pulses become high enough to trigger the circuit.
One approach which is known to overcome these problems uses two capacitors arranged in a capacitive voltage divider arrangement. This removes the secondary ringing problem and gives better timing than the secondary winding approach. However, this approach is problematic in that at least one high voltage capacitor is needed in the divider.
Another approach is to generate a blanking pulse from timing signals that precede the retrace interval. This can be done by using a pair of one-shot monostable multivibrators. A first one of the one-shots is triggered by one of the horizontal synchronizing signals and defines an initial delay of nearly one whole horizontal line. A second one-shot is triggered by the output of the first one-shot at the end of this delay, and defines the pulse width. However, there are problems associated one shots, such as false triggering, which results in improperly timed blanking.